Liquid level in a body of liquid, either in a vessel or outside a vessel, may be measured by any number of means including capacitance, ultrasonics, pneumatic float with magnet closing, multiple reed switches converted to resistance, mechanical floating arm changing the resistance of a rheostat and simple visual measurement of the liquid level on a scale.
Each of these methods, except simple visual measurement, are inferential measurements, that is to say, the electrical methods of measuring are analogue which are not as accurate as digital methods and suffer from the disadvantages of signal stability, drift, accuracy, calibration, error detection and size of range. Even visual measurement can suffer from reading errors.
Devices are available for optically or magnetically counting graduations to detect movement or to determine position. However, in the case of linear encoders, these graduations are not numbers themselves and the reading must be referenced to some starting point each time a reading is taken. In the case of rotary shaft encoders, the shaft position is a number read optically or magnetically, but the reading head remains stationary.
It is known that there are devices to read optical codes such as bar codes. In this case the optical reader may be moved over the bar coded item, or alternatively the optical reader is stationary and the bar coded item moves across the reader.